In a key step toward improving hurricane prediction, scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., have reproduced in a computer model the fine- scale structure that drives the birth and strengthening of tropical cyclones. NCAR's primary sponsor is the National Science Foundation (NSF), which also funded the research. NCAR scientists Jordan Powers and Christopher Davis are presenting imagery from their hurricane simulation this week in Fort Lauderdale, Florida, at the Ninth Conference on Mesoscale Processes, sponsored by the American Meteorological Society.
The simulation, which used the NCAR/Penn State (University) Mesoscale Model, Version 5 (MM5), marks the first time a cloud-resolving simulation has been able to reproduce the formation of a tropical cyclone, given only information about atmospheric conditions on a scale much larger than that of the cyclone. The breakthrough points toward future forecasting power that will soon be available. NCAR is part of a team now building a model similar to the MM5, but with more advanced capabilities, that will generate daily weather forecasts for the National Weather Service (NWS) beginning in 2004.
"Improved skill in forecasting in a research setting often does not quickly find its way into operational forecast models," says Cliff Jacobs, program director in NSF's division of atmospheric sciences. "This research has the best of all possible results: improved forecasting techniques that developed as a result of an investment in research, that likely will quickly make their way into operational models."
For their MM5 experiment, Davis and Powers studied Hurricane Diana, which struck North Carolina in 1984. Diana was chosen because of ample surface data and because a well-defined nontropical low preceded its formation. The MM5 successfully reproduced several stages in Diana's development, from its original state as a nontropical low to its intensification to hurricane status more than a day later.
According to Davis, "One of the remaining mysteries about hurricanes is how they form, especially when they're influenced by midlatitude weather systems that move into the subtropics and tropics. We hope that by analyzing the mechanisms behind storm formation in these simulations, we can make hypotheses of tropical cyclone formation that can be tested using aircraft, radar, and satellite data. We also hope to understand what's needed to predict storm formation in operational weather forecast models."
Computer models used for day-to-day weather prediction have become increasingly adept at projecting a hurricane's motion. Yet even the best models have little skill in predicting intensity, especially the rapid strengthening often noted in the most powerful hurricanes. Part of the problem is that the compact core of a hurricane, including the spiral bands of showers and thunderstorms that gather and focus energy, can’t be modeled in sufficient detail on the computers and models used for everyday forecasting.
The new Weather Research and Forecasting Model and more powerful computers will allow for the type of fine-scale detail in the MM5 to be simulated for daily forecasting. The National Oceanic and Atmospheric Administration, the University of Oklahoma, and the U.S. Air Force are collaborating with NCAR on the project.
"Diana" images available at: ftp://ftp.ucar.edu/communications
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